The main impetus of our work is to define the molecular parameters necessary for normal ovarian development in the mosquito, Aedes aegypti. Under ordinary circumstances females of this species are anautogenous, i.e., ovarian development is not initiated until after blood feeding. We have found that injection of the molting hormone ecdysone bypasses blood feeding in that initiation of ovarian development ensues. Furthermore, direct assay to determine the amount of the hormone endogenously has revealed an increase in the titer of ecdysone after a blood meal as compared to unfed females. No molting hormone was detected in the blood itself. We have concluded that after blood feeding ecdysone is released, activated or synthesized by adult females, by a mechanism as yet unknown.